1. Field of the Invention
The invention relates to an endoprosthesis, particularly a hip joint prosthesis, comprised of a metal shaft for being fitted into the bone and a ceramic head with a matching bore for receiving the free spigot of the metal shaft.
2. Description of the Prior Art
An endoprosthesis of this type is known, for example, from German Laid Open Specification No. 25 48 077. In the joint endoprosthesis described therein, the union between the metal shaft and the ceramic head is made in such a way that the free spigot of the metal shaft is shaped as a male cone while the bore in the ceramic head is shaped as a tapered conical socket. This produces a purely force locking connection when the two parts are pressed directly together. In order to enable use of relatively small cone angles to obtain a reliable friction connection, on the one hand and, on the other hand, to avoid the risk of cracking the ceramic head during heating of the endoprothesis for sterilization, that specification suggests providing grooves of minute profile depth on the male cone and/or in the tapered socket to produce deformation spaces between the metal shaft and the ceramic head into which the metal shaft can elastically expand. In this manner, the contact pressure in the socket of the ceramic head is intended to remain within certain tolerance limits to avoid the risk of cracking the inelastic ceramic head. It has been found, however, that this known technique of making a union by means of a self locking cone is still not sufficiently reliable for attaching a bio-ceramic hip joint head to the metal shaft, and it is particularly not reliable if the union is subjected to alternating loads.
From German Laid Open Specification No. 23 18 396 it is known to unite the metal shaft and cermaic head of a bone joint endoprosthesis by a screw connection. However, for reasons of cost of manufacture and for reasons of strength, a sufficiently fine thread for screwing inside the ceramic head cannot be used. Therefore, it has been suggested to insert a threaded metal sleeve into the ceramic head and to secure it in the head as an intermediate member between the ceramic head and the metal shaft. This made it possible to use a fine thread between the threaded sleeve and the metal shaft. But, there still were serious problems in securing the threaded sleeve inside the ceramic head. Furthermore, it proved necessary to provide additional constructional means to secure the screw connection. The use of an intermediate member between the metal shaft and the ceramic head and the need to secure the screw connection made the arrangement of this endoprosthesis so complicated that the idea of using a screw connection between a ceramic head and a metal shaft was again abandoned.